Historically, the French horn was a descendent of a hunting horn without valves but with the right hand placed in the bell to vary the pitch. When the orchestra changed key the hand horn was replaced by one of different length; twelve different horns were needed to play all twelve-keys.
The modern French horn is a compromise of two three-valve horns; the thumb valve changes the key from F to B flat. The result is a twelve-tone per octave instrument, but hand position and lip tension must be varied to obtain hand horn pitches in any key.